We don't have access to the internal representation of Dynamic, otherwise
we would not have to redefine it completely. Note that we use this only
internally, so the incompatibility with our Dynamic from the standard
Dynamic is not important.

The function unsafeCoerce# allows you to side-step the typechecker entirely. That
is, it allows you to coerce any type into any other type. If you use this function,
you had better get it right, otherwise segmentation faults await. It is generally
used when you want to write a program that you know is well-typed, but where Haskell's
type system is not expressive enough to prove that it is well typed.

The following uses of unsafeCoerce# are supposed to work (i.e. not lead to
spurious compile-time or run-time crashes):

Casting any lifted type to Any

Casting Any back to the real type

Casting an unboxed type to another unboxed type of the same size
(but not coercions between floating-point and integral types)

Casting between two types that have the same runtime representation. One case is when
the two types differ only in "phantom" type parameters, for example
Ptr Int to Ptr Float, or [Int] to [Float] when the list is
known to be empty. Also, a newtype of a type T has the same representation
at runtime as T.

Other uses of unsafeCoerce# are undefined. In particular, you should not use
unsafeCoerce# to cast a T to an algebraic data type D, unless T is also
an algebraic data type. For example, do not cast Int->Int to Bool, even if
you later cast that Bool back to Int->Int before applying it. The reasons
have to do with GHC's internal representation details (for the congnoscenti, data values
can be entered but function closures cannot). If you want a safe type to cast things
to, use Any, which is not an algebraic data type.